In the manufacture of paper, an aqueous cellulosic suspension or slurry, is formed into a paper sheet. The slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1%, and often below 0.5%, ahead of the paper machine, while the finished sheet must have less than 6 weight percent water. Hence the dewatering aspects of papermaking are extremely important to the efficiency and cost of manufacture.
The least costly dewatering method is drainage, and thereafter more expensive methods are used, including vacuum pressing, felt blanket blotting and pressing, evaporation and the like, and any combination of such methods. Because drainage is both the first dewatering method employed and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other methods and improve the overall efficiency of dewatering, thereby reducing the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost of manufacture is the retention of furnish components on and within the fiber mat being formed. The papermaking slurry represents a system containing significant amounts of small particles stabilized by colloidal forces. A papermaking furnish generally contains in addition to cellulosic fibers, particles ranging in size from about 5 to about 1000 nanometers consisting of, for example, cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would pass through the spaces (pores) between the cellulosic fibers in the fiber mat being formed.
Greater retention of fines, fillers, and other slurry components permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of lower quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes even more important because the fines content of such lower quality pulps is generally greater than that of pulps of higher quality. Greater retention also decreases the amount of such substances lost to the white water and hence reduces the amount of material wastes, the cost of waste disposal and the adverse environmental effects therefrom. It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation may be determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90%, the formation parameter generally declines.
Various chemical additives have been utilized in an attempt to increase the rate at which water drains from the formed sheet, and to increase the amount of fines and filler retained on the sheet. The use of high molecular weight water soluble polymers was a significant improvement in the manufacture of paper. These high molecular weight polymers act as flocculants, forming large flocs which deposit on the sheet. They also aid in the dewatering of the sheet. In order to be effective, conventional single and dual polymer retention and drainage programs require incorporation of a higher molecular weight component as part of the program. In these conventional programs, the high molecular weight component is added after a high shear point in the stock flow system leading up to the headbox of the paper machine. This is necessary because flocs are formed primarily by the bridging mechanism and their breakdown is largely irreversible and do not re-form to any significant extent. For this reason, most of the retention and drainage performance of a flocculant is lost by feeding it before a high shear point. On the other hand, feeding high molecular weight polymers after the high shear point often leads to formation problems. Thus, the feed requirements of the high molecular weight polymers and copolymers which provide improved retention often lead to a compromise between retention and formation. Accordingly, inorganic "microparticles" were developed and added to high molecular weight flocculant programs to improve performance.
Polymer/microparticle programs have gained commercial success replacing the use of polymer-only retention and drainage programs in many mills. Microparticle-containing programs are defined not only by the use of a microparticle component, but also often by the addition points of chemicals in relation to shear. In most microparticle-containing retention programs, high molecular weight polymers are added either before or after at least one high shear point. The inorganic microparticulate material is then usually added to the furnish after the stock has been flocculated with the high molecular weight component and sheared to break down those flocs. The microparticle addition re-flocculates the furnish, resulting in retention and drainage that is at least as good as that attained using the high molecular weight component in the conventional way (after shear), with no deleterious impact on formation.
One such program employed to provide an improved combination of retention and dewatering is described in U.S. Pat. Nos. 4,753,710 and 4,913,775, the disclosures of which are incorporated herein by reference. In accordance with these patents, a high molecular weight linear cationic polymer is added to the aqueous cellulosic papermaking suspension before shear is applied to the suspension, followed by the addition of bentonite after the shear application. Shearing is generally provided by one or more of the cleaning, mixing and pumping stages of the papermaking process, and the shear breaks down the large floes formed by the high molecular weight polymer into microflocs. Further agglomeration then ensues with the addition of the bentonitc clay particles.
Other such microparticle programs are based on the use of colloidal silica as a microparticle in combination with cationic starch such as that described in U.S. Pat. Nos. 4,388,150 and 4,385,961, the disclosures of which are incorporated herein by reference, or on the use of a cationic starch, flocculant, and silica sol combination such as that described in U.S. Pat. Nos. 5,098,520 and 5,185,062, the disclosures of which are also incorporated herein by reference. U.S. Pat. No. 4,643,801 discloses a method for the preparation of paper using a high molecular weight anionic water soluble polymer, a dispersed silica, and a cationic starch.
Although, as described above, the microparticle is typically added to the furnish after the flocculent and after at least one shear zone, the microparticle effect can also be observed if the microparticle is added before the flocculent and the shear zone (e.g., wherein the microparticle is added before the screen and the flocculent after the shear zone).
In a single polymer/microparticle retention and drainage aid program, a flocculant, typically a cationic polymer, is the only polymer material added along with the microparticle. Another method of improving the flocculation of cellulosic fines, mineral fillers and other furnish components on the fiber mat using a microparticle is in combination with a dual polymer program which uses, in addition to the microlarticle, a coagulant and flocculant system. In such a system a coagulant is first added, for instance a low molecular weight synthetic cationic polymer or cationic starch. The coagulant may also be an inorganic coagulant such as alum or polyaluminum chlorides. This addition can take place at one or several points within the furnish make up system, including but not limited to the thick stock, white water system, or thin stock of a machine. This coagulant generally reduces the negative surface charges present on the particles in the furnish, particularly cellulosic fines and mineral fillers, and thereby accomplishes a degree of agglomeration of such particles. The coagulant treatment is followed by the addition of a flocculent. Such a flocculant generally is a high molecular weight synthetic polymer which bridges the particles and/or agglomerates, from one surface to another, binding the particles into larger agglomerates. The presence of such large agglomerates in the furnish, as the fiber mat of the paper sheet is being formed, increases retention. The agglomerates are filtered out of the water onto the fiber web, whereas unagglomerated particles would, to a great extent, pass through such a paper web. In such a program, the order of addition of the microparticle and flocculant can be reversed successfully.
The present invention departs from the disclosures of these patents in that an anionic nanocomposite is utilized as the microparticle. As used herein, nanocomposite means the incorporation of an anionic polyelectrolyte into the synthesis of a colloidal silica. Nanocomposites are known in other fields/have been used in other applications, such as ceramics, semiconductors and reinforced plastics.
The present inventors have surprisingly discovered that anionic nanocomposites provide improved performance over other microparticle programs, and especially those using colloidal silica sols as the microparticle. The anionic nanocomposites of the invention exhibit improved retention and drainage performance in papermaking systems.